Semiconductor nanocrystals, or “quantum dots,” are particles whose radii are smaller than the bulk exciton Bohr radius and constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of semiconductor nanocrystals shift to the blue (higher energies) as the size of the nanocrystals gets smaller.
Quantum dots are composed of an inorganic, crystalline, semiconductive material and have unique photophysical, photochemical, and nonlinear optical properties arising from quantum size effects, and have therefore attracted a great deal of attention because of their potential applicability in a variety of contexts, e.g., in biological detection, light-to-chemical or light-to-electrical energy conversion schemes, catalysis, displays, and telecommunications. Quantum dots are characterized by size-dependent properties such as peak emission wavelength and quantum yield. These crystals generally vary in size from about 1 nm to 100 nm and may be variously composed of elements, alloys, or other compounds. The desirable properties of quantum dots differ depending on the field of use, but a “tunable” peak emission wavelength, chemical stability, and photochemical stability are generally viewed as very important regardless of context.
For emission in the visible region of the electromagnetic spectrum, cadmium selenide (CdSe) materials have by far been the most important class of quantum dots, largely because they exhibit size-dependent luminescence tunable throughout the visible wavelength range. That is, by changing the particle size of CdSe quantum dots, the emission can be varied throughout most of the visible wavelength region. Proper selection of synthetic conditions furthermore allows the preparation of exceptionally bright quantum dots with luminescence efficiencies approaching unity (i.e. one emitted photon for every absorbed photon). Unfortunately, these CdSe-based quantum dots suffer from less than ideal stability characteristics, particularly with regard to chemical degradation and photooxidation. Emission from the nanocrystals is fairly easily and irreversibly quenched under conditions common in, for example, biological assays and biomedical labeling applications.
A key innovation that has significantly increased the utility of quantum dots is the addition of a discrete inorganic shell over the nanoparticle core. That is, decomposition pathways in many quantum dots, including CdSe nanocrystals, usually involve the formation of defects known as traps on the surfaces of the quantum dots. The key to ensuring and maintaining quantum dot emission is to passivate these surface sites. Some have had reasonable success in passivating nanocrystal surfaces using organic capping materials such as an alkylamine or trioctylphosphine oxide, but thus far these approaches have proven inadequate, dramatically decreasing luminescence intensity and resulting in nanoparticles that are insufficiently robust for many applications including biological detection. The use of inorganic compounds as capping agents has proven far more successful, providing that the material used is optically non-interfering, chemically stable, and lattice-matched to the underlying material. This last property is particularly important, since matching the lattices, i.e., minimizing the differences between the shell and core crystallographic structure, minimizes the likelihood of local defects, shell cracking, and formation of long-range defects. Typically, a large band gap semiconducting material such as zinc sulfide (ZnS) will be used to epitaxially overcoat nanocrystal cores with a crystalline shell that matches the underlying lattice. In other words, crystalline growth of a core material such as CdSe can be halted and then continued using a related crystalline material such as ZnS to form the shell. While this outer material doesn't necessarily contribute directly to the size-tunable properties of interest such as peak emission wavelength, such a passivating layer can have a substantial indirect impact. For example, the brightness of core-shell materials often far exceeds that of base nanoparticle core materials. Additionally, resistance to chemical and photochemical decomposition is often markedly increased.
Though not often recognized, such shell chemistry can be critically important to the utility of quantum dots, at least in applications that require certain stable properties such as predictable non-fluctuating emission characteristics. Indeed, it has been the invention of the core-shell concept that has resulted in recent attempts to commercialize quantum dot technology in several fields, including biotechnology and solar energy applications.
Not only is the composition of the shell of central importance to the characteristics of the final quantum dot, but the method of depositing the shell material is important as well. High quality inorganic shells must be thick enough to be sufficiently protective, and, ideally, are intimately wed to the underlying core. The reason for the latter requirement is that core crystals and shell crystals seldom have completely matched lattice spacings. For example, with a CdSe/ZnS nanoparticle as described above, the ZnS shell is characterized by shorter average bond lengths than in the CdSe core. In order to successfully form high quality composite structures, special precautions must be taken, e.g., doping of atoms from the core into the shell to relax the lattice in the shell and allow it to more easily match the lattice of the core. The use of these alloyed or mixed shells has been described in U.S. Pat. No. 6,815,064 to Treadway et al., assigned to Quantum Dot Corporation (Hayward, Calif.) and incorporated by reference herein.
Despite the significant commercial impact of the new engineered nanoparticle structures, there remain limitations in the field which have not yet been overcome. For example, it is now understood that many applications demand the ability to independently tune the size and the emission characteristics of the final nanoparticles rather than allowing them to move in lockstep. One example of this need is related to the fact that the efficiency of light absorption, and therefore the ultimate brightness, of the nanoparticle is a steep function of the particle size. It is desirable therefore in some applications requiring extremely sensitive detection to maximize the size of the nanoparticles without necessarily maximizing the emission wavelength for the resulting labels. Alternatively, some potential uses for quantum dots require them to be prepared as small as possible, particularly where steric or other physical constraints limit the size of the label which can be used (e.g., labeling inside the nuclei of living cells). Again, it is desirable to prepare a palette of colors for this application, but here smaller particles are more useful.
Another limitation which must be overcome is the fact that the cost of research associated with the development of high-quality inorganic passivating layers for nanoparticles is extremely high. Passivation is critical to most quantum dot applications, if not all applications requiring luminescent versions of the particles. For this reason, it is useful to make a single overcoating material (e.g., ZnCdS) serve for many distinct nanocrystal core compositions.
It has further proven difficult to engineer the peak emission wavelength of a luminescent semiconductor nanoparticle, e.g., to provide emission in the red to infrared regions of the spectrum, without increasing particle size.
Some attempts have been made to engineer the electronic and optical properties of luminescent semiconductor nanoparticles by doping nanoparticle cores with an additive. For instance, a method has been described for increasing peak emission wavelength by doping nanoparticle cores with a material capable of shifting the emission peak to the desired extent, but the resulting core-shell structures exhibit rapid degradation of optical properties. See, e.g., Zhong et al. (2003), “Composition-Tunable ZnxCd1-xSe Nanocrystals with High Luminescence and Stability,” J. Am. Chem. Soc. 125:8589-8594. This is in large part because the amount of dopant believed necessary to effect a significant change in peak emission wavelength was so high, and an introduction of a substantial amount of dopant changed the properties of the core adversely. For example, it has been disclosed that doping CdSe with Te at a level of at least 50% (such that the molar ratio of Te to Se in the CdSe1-xTeX core is greater than 1:1) is necessary to provide a meaningful increase in peak emission wavelength. See Bailey et al. (2003), “Alloyed Semiconductor Quantum Dots: Tuning the Optical Properties without Changing the Particle Size,” J. Am. Chem. Soc. 125:7100-7106.
There remains, accordingly, a need for a way to prepare luminescent nanoparticles in a manner that enables engineering of key electronic, optical, and physical properties, e.g., bandgap energy, brightness, peak emission wavelength, chemical stability, and photochemical stability, without necessarily increasing particle size or significantly changing the composition of the nanoparticle core. Such a method would be highly valuable in many contexts, for example enabling preparation of bright, stable particles emitting in a longer wavelength region—e.g., in the red to infrared regions of the electromagnetic spectrum.